Apparatus for coloring articles, for instance lens for spectacles

ABSTRACT

An apparatus for coloring lens for spectacles by evaporating the coloring material under vacuum, comprises a vacuum chamber where both the lens to be colored and the coloring material are introduced. The material is evaporated by a resistor which is supplied with a constant voltage by a variable transformer. The transformer is disabled by a comparing circuit apt to compare the intensity of coloration, continuously sensed by a photocell, with a predetermined intensity of coloration previously set on a potentiometer, upon said photocell sensing a sample lens.

United States Patent Tassara Apr. 2, 1974 [54] APPARATUS R COLORING ARTICLES, 2,927,501 3 1960 Busignies et al. 118/90 x FOR INSTANCE LENS FOR SPECTACLES 2,978,364 4/1961 Blaustein 1 18/8 X 3,059,611 10/1962 Fury ct al. 118 8 Inventor: Pmo a, Mllano, Italy 3,382,842 5 1968 Steckelmacher ct a1. 118 8 3,383,238 5 1968 Unzicker ct al. 118/90 x [73] Asslgnee' Metal Italy 3,526,460 9 1970 Webb 118/9 x [22] Fil d;' J l 24 1972 3,600,675 8/1971 Grenier 118]) X 3,670,693 6/1972 Rorick et al, 118 8 x Appl. No.: 274,688

Related [1.5. Application Data Continuation of Ser. No. 139,035, April 30, 1971, abandoned.

Foreign Application Priority Data May 6, 1970 Italy 24220 70 US. Cl. 118/8, 118/49 Int. Cl. C23c 13/08 Field of Search 118/4, 7, 8, 48-495, 118/9, 11; 250/219 TH, 226; 356/202, 203, 160, 161, 205, 206; ll7/93.3

References Cited UNITED STATES PATENTS 12/1952 Chilowsky 118/8 Primary Examiner-Morris Kaplan [57] ABSTRACT An apparatus for coloring lens for spectacles by evaporating the coloring material under vacuum, comprises a vacuum chamber where both the lens to be colored and the coloring material are introduced. The material is evaporated by a resistor which is supplied with a constant voltage by a variable transformer. The transformer is disabled by a comparing circuit apt to compare the intensity of coloration, continuously sensed by a photocell, with a predetermined intensity of coloration previously set on a potentiometer, upon said photocell sensing a sample lens.

12 Claims, 4 Drawing Figures Pmm mra 2mm (800,738

FIG.1

PAIENTEDA'PR 2 I974 SHEEI 2 (IF 4 PATENTED 2 5 SHEET 3 [IF 4 FIG.3

APPARATUS FOR COLORING ARTICLES, FOR INSTANCE LENS FOR SPECTACLES CROSS REFERENCE TO RELATED APPLICATION This application is a continuation application of application Ser. No. 139,035, filed Apr. 30, 1971, now abandoned entitled: APPARATUS FOR COLORING ARTICLES, FOR INSTANCE LENS FOR SPECTA- CLES.

Applicant claims priority from corresponding Italian patent application Ser. No. 24220-A/70, filed May 6, 1970.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus for coloring varticles, for instance lens for spectacles, by evaporating the coloring material under vacuum. More particularly, the invention relates to an apparatus for coloring lens in the laboratory or shop of an optician, who has to color the lens required by a customer according to a color and an intensity of color chosen by the customer, whereby normally very few lens at a time are to be colored.

2. Description of thePrior Art Known apparatus of the above type normally comprise a vacuum chamber where the lens and the coloring material are introduced. The coloring material is then heated by means of an electric resistor until it is evaporated. The electric power supplied to the resistor and the time of the process is normally controlled manually, according to either the feeling and skill of the operator or to some predetermined factors. The result of the process is thus unreliable, whereas the apparatuses are intricate in operation because of the manual knobs.

SUMMARY OF THE INVENTION The main object of the invention is to provide an apparatus for coloring lens obviating the above disadvantages and achieving an absolutely constant result of the process.

Another object of the invention is to provide an apparatus of the above type, wherein the heating of the coloring material is automatically controlled both as intensity and as time.

According to the invention in an apparatus for coloring articles, for example lens for spectacles, by evaporating a coloring material under vacuum, comprising a vacuum chamber where an article to be colored and said coloring material are introduced, and heating means located inside said chamber for heating said coloring material so as to evaporate, I now provide comparing means for comparing the intensity of coloration of the article being colored with a predetermined intensity of coloration, and control means conditioned by said comparing means for controlling said heating means.

It is evident that the apparatus is automatically controlled, thus saving a considerable amount of manpower. The operator has merely to load and unload the apparatus with the lens and the coloring material.

In fact, whereas the loading operation requires only a few minutes, the entire cycle of the process requires about half an hour, whereby a single operator can load sequentially a lot of apparatuses, the process of which is then effected automatically.

The above characteristic will become more clear from the following description of a preferred embodiment of the invention, and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sectional vertical view of an apparatus for coloring lens for spectacles, according to the invention;

FIG. 2 is another partial sectional vertical view of the apparatus of FIG. 1 in an enlarged scale;

FIG. 3 is a plan sectional view taken according to a pair of different planes of FIG. 2;

FIG. 4 is a diagram of the electric circuits controlling the apparatus.

DETAILED DESCRIPTION OF THE INVENTION With reference to FIG. 1, the numeral 4' generically indicates the stationary frame of the apparatus, which is formed as a box. The frame 4 supports upwards a circular plate 5, which is the base of a vacuum chamber 6. The plate 5 is provided with a hole 7 housing an electrovalve, generically indicated by the numeral 11. The valve 11 is connected by means of a duct 8 to a vacuum pump 9, adapted to generate a high degree of vacuum. The pump 9 is of any known type, for instance of the type using the diffusion of silicon vapor, and is therefore controlled by the valve 11.

Another pump 10 adapted to generate a low degree of vacuum, for instance of the rotative type, is connected to an electric motor 12, whereas a group of electrovalves 13 is operable to connect selectively the chamber 6 with the pump 10, or even with the external space, or the pump 10 with the pump 9.

A vertical shaft 14 is rotatably mounted on the plate 5 so as to have with this latter an airtight seal. The shaft 14 is connected downwards with an electric motor 15 provided with a conventional motion reducing gear mechanism. The shaft 14 is secured upwards to thecentral portion of a bar 16 located inside the chamber 6. A pair of similar crucibles l7 and 18 are secured to the two ends of the bar 16. The crucible I7 is provided to support a coloring material, for instance formed of a mixture of metal oxides, whereas the crucible 18 is provided to support an antireflective material, for instance formed of calcium fluoride. I The crucible 17 is normally located under an electric resistor 19, which is supported by a rod of insulated material 71 crossing the plate 5 in an airtight manner. The rod 71 projects below the plate 5 and is crossed by electric cables connecting the resistor 19 to a variable transformer 33. This latter is operated by a reversible electric motor 34 known per se, so as to increase the power supplied by the transformer 33 when the motor 34 is rotated in one direction and to decrease such a power when the motor 34 is rotated in the opposite direction, the motor 34 being almost rotated 360. The rod 71 can be refrigerated by means of a liquid circulating through a pair of ducts 76'.

Secured to the plate 5 is also another rod 74 (FIG. 2), which is electrically connected with a power supply unit 3 (FIG. 1). The rod 74 is adapted to generate a high voltage discharge under the control of a suitable electric circuit not shown in the drawing.

Secured to the plate 5 is also a frame generically indicated by the numeral 120 and formed of a pair of vertical rods connected by a transverse bar 20. Secured to the central part of the bar 20' is a frusto-conic support 21 the lateral surface of which carries a plurality of holed seats 21' (FIG. 2) each one adapted to house one lens 22 to be colored. Located under the support 21 (FIG. 1) is another resistor 1 19, which is connected by a cable 171 to the power supply unit 3 and can be switched on to bring the chamber 6 at a predetermined temperature in a manner to be described.

The chamber is provided upwards with an eccentrically located hole 123 (FIG. 2), which connects the inner space of the chamber 6 with a first tube 24 secured to chamber 6 in an airtight manner. A second tube 24', secured to tube 24 by means of a pair of flanges 24", is tightly sealed downwards by means of a transparent glass 23.

The tube 24' is provided downwards with a lens 25 adapted to concentrate a light beam downwards, in a manner known per se. Secured to the upper end of the tube 24' is a light source formed of a lamp 26, which is supplied with a constant voltage. The tube 24 is also provided with apair of horizontal slots 29 located between the lamp 26 and the lens 25. The slots 29 (FIG. 3) slidably mount a slide 28 having a pair of similar seats 27 one of which normally supports an uncolored lens 30, whereas the other supports a colored sample lens 31. Normally one of lens 30 and 31 is located within the tube 24, whereas the other is outside the tube 24 (in FIGS. 2 and 3 the lens 30 is outside the tube 24').

The apparatus comprises a program control unit generically indicated by the numeral 132 (FIG. 1) and cyclically operable in a known manner for controlling the cyclic operation of the apparatus. More particularly, the program control unit 132 comprises a shaft 32 cyclically rotatable so as to effect one revolution in half an hour and carrying a set of cams, a cam 110 which is shown in FIG. 1. These cams are adapted to operate an associated set of switches to control in a predetermined sequence the various operating phases of the cycle. Particularly, a first cam 36 (FIG. 4) of the shaft 32 normally holds a switch open. The switch 35 when closed causes the reversible motor 34 to rotate in the direction to increase the power supplied by the transformer 33. Another cam 72 of the shaft 32 controls a second switch 37, which at rest is also open. The switch 37 when closed is adapted to cause the reversible motor 34 to be rotated in the opposite direction to progressively reduce the power supplied by the transformer 33 until it dissappears completely.

In order to control the power supplied by the transformer 33, the apparatus is provided with an automatic control device which comprises a wattmetric circuit 38 supplied through connectors A and B with the same current and voltage supplied to the resistor 19. The control device also comprises a comparing and amplifying circuit, generically indicated by the numeral 39, and formed of a node 46 connected through a resistor 47 to a potentiometer 48. This latter is connected between a negative polarity Va and ground. Connected in parallel between the node 46 and another node 49 are a linear amplifier 50 and a feedback resistor 51.

The control device comprises also a threshold circuit generically indicated by the numeral and formed of a resistor 53 connected between the node 49 and the base of a transistor 52. This latter is in a nonconductive state as long as the voltage on the node 49 is less than threshold voltage of 0.6V, but it becomes conductive when the voltage on the node 49 reaches or is higher than 0.6 V. The transistor 52 is connected to an electric relay R1, which controls an electric contact r1. This latter is normally closed and is seriesconnected with the switch 35.

The apparatus is provided with a second automatic control device, comprising a photometric circuit, generically indicated by the numeral 55. This latter is formed of a photoelectric cell 58 located within a protected tube 44 (FIG. 2), which is secured to the plate 5 and is vertically aligned with the lamp 26. The photocell 58 is adapted to generate a voltage proportional to the luminous intensity received from the lamp 26 through the coloring lens 22 and either the uncolored lens 30 or the colored sample lens 31.

The second control device also comprises a comparing and amplifying circuit generically indicated by the numeral 56. This latter is formed of a resistor 59 connected between the photocell 58 and a node 60. A resistor 61 is connected between the node 60 and a potentiometer 62 in turn connected between the negative voltage source Va and ground. Furthermore, connected in parallel between the node 60 and another node 63 are a linear inverter amplifier 64 and a feedback resistor 65.

The second control device comprises also control means formed of a threshold circuit 57 having a resistor 66 connected to the base of a transistor 67, which is adapted to become conductive when the voltage in the node 63 reaches, or is higher than, the threshold voltage of 0.6 V. The transistor 67 is connected with an electric relay R2, which controls a corresponding contact r2 series-connected with the switch 37. An alarm lamp 68 is also connected in parallel with the relay R2 and therefore is fed by the same transistor 67.

Finally, the apparatus comprises a manipulative knob 69 (FIG. 1) for starting a cyclic operation of the apparatus, a knob 78 for calibrating the photometric circuit 55 (FIG. 4) and a knob 76 (FIG. 1) for switching on the apparatus.

The coloring apparatus is operated as follows.

When some lens 22 are to be colored, the chamber 6 is removed from the plate 5 and the lens 22 to be colored are inserted on the seats 21' (FIG. 2) of the support 21. Furthermore, the crucible 17 (FIG. 1) is loaded with the coloring material and, in the case the antireflective treatment is required, the crucible 18 is loaded with the antireflective material. Then, in case the crucible 17 is not aligned with the resistor 19, the bar 16 is manually rocked so as to exactly align vertically the crucible 17 with the resistor 19. Now the chamber 6 is placed on the plate 5 and the colored sample lens 31 (FIG. 2) is inserted on the seat 27 of the slide 28, which is then moved so as to bring the sample lens 31 between the lamp 26 and the lens 25 in the position shown in FIG. 2.

Now the photometric circuit 55 (FIG. 4) is calibrated. To this end, the apparatus is switched on by operating the knob 76, (FIG. 1) which causes the lamp 26 to be switched on. The light beam crosses the sample lens 31 (FIG. 2), the lens 25, and the lens 22 to be colored, the lens 25 concentrating the beam to the photoelectric cell 58. This latter generates thus a voltage proportional to the received light intensity and having a positive polarity as shown in FIG. 4.

Then, by operating the knob 78 (FIG. 1) in a suitable manner, the potentiometer 62 (FIG. 4) is preset or adjusted so as to cause the current in the node 60, resulting in the current crossing the resistor 59 and in the current crossing the resistor 61, upon being amplified by the amplifier 64 to produce on the node 63 the threshold voltage of 0.6 V. This condition is achieved when the lamp 68 is just switched on. The comparing and amplifying circuit 56 is thus calibrated.

To effect the coloring cycle of the lens 22 (FIG. 1), the slide 28 must be shifted so as to bring under the lamp 26 the uncolored lens 30. The lamp 26 causes now an increasing of the current crossing the resistor 59 (FIG. 4), whereby the relay R2 is deenergized and the contact r2 is open. Now the starting knob 69 is operated, thus starting the cycle of the program control unit 132 (FIG. 1) and therefore causing one revolution of the shaft 32. At rest the two cams 36 and 72 (FIG. 4) hold the two switches 35 and 37 open, whereby the motor 34 is standing and the transformer 33 sends no current to the resistor .19 (FIG. 1). At the beginning of the cycle, the shaft 32 through the cam 110 causes the motor 12 to operate the pump during about 5 minutes. The pump 10 produces into the chamber 6 a relatively low vacuum of about 10 mm of mercury. Then the shaft 32 through another cam, not shown in the drawings, causes the resistor 119 to preliminary heat the space inside the chamber 6. The resistor 119 will remain switched on during the remaining part of the cycle to maintain a predetermined temperature inside the chamber 6.

Thereafter the shaft 32 through a further cam, not shown in the drawings, causes in a known manner the rod 74 (FIG. 2) to produce a high voltage discharge in the chamber 6. The effect of this discharge is to cause the gas absorbed by the walls of the chamber 6 and the plate 5 to expand and to leave the latters, thus improving the conditions for producing the vacuum in the chamber 6. Now the shaft 32 causes the electrovalve 1 l to connect the chamber 6 with the pump 9 at high degree of vacuum, which is thus operated during about minutes. The pump 9 reduces thus the pressure in the chamber 6 to 10' mm of mercury.

Now the cam 36 (FIG. 4) closes the switch 35, which causes the motor 34 to rotate in such a direction as to cause the transformer 33 to increase the power supplied to the resistor 19. The heating of the resistor 19 is thus started and this latter heats also the crucible 17 (FIG. 1). The power absorbed by the resistor 19, through the wattmetric circuit 38 (FIG. 4), produces a positive current proportional to this power; this current is applied to the node 46. A negative current generated by the polarity Va through the potentiometer 48 and the resistor 47 is also applied to the node 46, whereby the resulting current, that is the algebraic sum of the two currents, is applied to the amplifier 50.

At the beginning the resulting current is negative. As the electric power absorbed by the resistor 19 increases, the negative resulting current decreases until the current coming out from the circuit 38 becomes equal to that coming out from the potentiometer 48. Thereafter the resulting current becomes positive and increases progressively. The resulting current through the amplifier 50 reaches the node 49, which presents thus a voltage prportional to the resulting current. This 6 voltage through the resistor 53 is appliedto the base of the transistor 52.

When the voltage at the base of the transistor 52 reaches the threshold voltage 0.6 V, the transistor becomes conductive and energizes the relay R1. This latter opens the contact rl, thus stopping the motor 34, whereby the transformer 33 supplies now a constant power to the resistor 19. Since the threshold voltage corresponds to the vaporizing temperature of the coloring material, this latter is now evaporated, whereas the crucible 17 is now heated by the resistor l9 at said constant power, whereby the evaporization is effected at a constant speed. In the meantime the cam 36 opens temporarily the switch 35. The coloring material thus evaporated deposits on the lower surface of the lens 22, by sublimation due to the lower temperature thereof with respect to the crucible 17, whereby the lenses 22 are progressively colored.

Now the cam 72 of the shaft 32 closes the switch 37, but the relevant circuit is not closed becasue the relays R2 is deenergized and therefore the contact r2 is open.

Due to the coloration of the lens 22 (FIG. 2), the photocell 58 reduces now progressively the current generated thereby, according to the reduction of the light intensity crossing the lens 22 aligned with the photocell 58. When the lens 22 reach the intensity of coloration of the sample lens 31, the algebraic sum of the currents crossing the node 60 (FIG. 4), through the inverter amplifier 64, the node 63 and the resistor 65 produces at the base of the transistor 67 the threshold voltage of 0.6 V and the transistor 67 becomes conductive.

Now the relay R2 is energized, thus closing the contact r2, this latter causes the motor 34 to rotate in the opposite direction so as to rapidly reduce the power supplied by the transformer 33. The evaporization of the coloring material is thus stopped, while the transistor 52 becomes conductive and deenergizes the relays R1, whereby the contact r1 is closed again. Finally, the cam 72 of the shaft 32 causes the switch 37 to open, whereby the motor 34 is stopped.

The shaft 32, through another cam, not shown in the drawings, starts now the motor 15, which rocks the shaft 14 together with shaft the bar 16 whereby the crucible 18 is now brought under the resistor 19. The cam 36 closes new again the switch 35, whereby the motor 34 is rotated so as to cause the transformer 33 to supply an increasing power to the resistor 19. The motor 34 is then arrested when the power supplied by the transformer reaches the predetermined value, in the manner above described.

Now the antireflective material is evaporated at a constant rate. This evaporation is automatically stopped after a predetermined delay under the control of the shaft 32, which causes the cam 36 to open the switch 35 and the cam 72 to close again temporarily the switch 37. The motor 34 is thus rotated as to rapidly reduce the power supplied by the transformer 33, the shaft 14 (FIG. 1) is rocked again 180 degrees, whereas the valve 13 is operated to restore the pressure into the chamber 6, and the cycle of the shaft 32 is stopped. The operator raises now the chamber 6 and removes the colored lens 22 from the support 21.

It is to be noted that the calibration of the photoelectric control device 55 can be effected also during the coloring cycle, provided that it will finish before the be ginning of the evaporation of the coloring material.

From the above description, it will be clear that the comparing means 56 are adapted to compare the coloring intensity of the lens 22 with the coloring intensity of the sample lens 31 and that the control means 57 are conditioned by the comparing means 56 for controlling the heating means 19.

What l claim is:

1. An apparatus for coloring lens for spectacles by evaporating a coloring material under vacuum, comprising a vacuum chamber where both the lens to be colored and said coloring material are introduced, and heating means located inside said chamber for heating said coloring material so as to evaporate this latter, wherein the improvement comprises: a photometric control device for directly sensing the transparency of the lens being colored to generate an electric current proportional to the luminous intensity of the lens being colored, means operable before heating said coloring material for causing said photometric control device to directly sense temporarily the transparency of a sample lens to generate a reference electriccurrent, an element presettable according to said reference electric current and connected to an electric power source so as to generate an electric current proportional to the luminous intensity of the sample lens, a comparing circuit for comparing the current generated by said element so preset with the current generated by said photometric control device in sensing said lens being colored, and control means conditioned by said comparing circuit for controlling said heating means.

2. An apparatus according to claim 1, wherein said element is formed of a potentiometer, and wherein said photometric control device comprises a photocell, said potentiometer being preset by hand upon sensing said sample lens throgh said photocell.

3. An apparatus according to claim 2, wherein said operable means comprises a shifting member shiftable between two different positions, said member having a pair of seats, one of which is loaded with a transparent lens and the other with the sample lens said member when in one of said positions locating said transparent lens in correspondence with said photocell, when in the other of said positions locating said sample lens in correspondence with said photocell.

4. An apparatus according to claim 1, wherein said comparing circuit comprises a threshold circuit, which operates said control means to switch off said heating means.

5. An apparatus according to claim 1, wherein said heating means comprises a resistor,. and a variable transformer controlled by a wattmetric control circuit connected to said resistor for causing said transformer to supply a constant power to said resistor.

6. An apparatus according to claim 5, comprising a reversible motor connected to said transformer and operable in a first direction to increase the power supplied by said transformer and in a second direction to decrease said supplied power, said wattmetric control circuit causing said motor to be rotated in said first direction, said wattmetric control circuit comprising a second comparing circuit for comparing a predetermined electric current with a current proportional to the electric power absorbed by said resistor to stop said motor when a predetermined power is absorbed by said resis tor.

7. An apparatus according to claim 6, wherein said second comparing circuit comprises a first threshold circuit for opening said motor in said first direction, said predetermined electric current being produced by a potentiometer preset so as to generate. a corresponding threshold voltage when said predetermined power is absorbed.

8. An apparatus according to claim 7, wherein said control means comprises a second threshold circuit to cause said motor to rotate in said second direction, said first and second threshold circuits operating said motor by means of two corresponding relays.

9. An apparatus according to claim 1, comprising a pair of crucibles, one of which is loaded with said coloring material, the other of which is loaded with an antireflective material, a common support for carrying both said crucibles, and means for alternately shifting said crucibles in correspondence with said resistor.

10. An apparatus according to claim 9, wherein said common support comprises a shaft rotatable stepwise and a transverse bar carrying said crucibles at the two ends thereof and having the middle portion secured to said shaft.

11. An apparatus according to claim 10, comprising a programcontrol unit for sequentially controlling the operations of said motor and the stepwise rotation of said shaft, said control unit controlling also at least a vacuum pump and high voltage discharge means inside said chamber.

12. Apparatus for coloring articles, for example lens for spectacles, by evaporating a coloring material under vacuum, comprisng a vacuum chamber where both an article to be colored and said coloring material are introduced, and heating means located inside said chamber for heating said coloring material so as to evaporate this latter, wherein the improvement comprises comparing means for comparing the intensity of coloration of the article being colored with a predetermined intensity of coloration, control, means conditioned by said comparing means for controlling said heating means, said comparing means comprising a comparing circuit for comparing a current proportional to the luminous intensity of the article being colored and generated by a photometric control device, with a current generated by an element preset according to said predetermined intensity, said articles being made of glass, and a shifting member shiftable between two different positions, said member having a plurality of seats, one of which is loaded with a transparent article and another with the sample article, said member when in one of said positions locating said transparent article in correspondence with said photometric control device and when in the other of said positions locating said sample article in correspondence with said photometric control device, whereby said predetermined intensity may be determined by sensing the sample article through said photometric control device. 

1. An apparatus for coloring lens for spectacles by evaporating a coloring material under vacuum, comprising a vacuum chamber where both the lens to be colored and said coloring material are introduced, and heating means located inside said chamber for heating said coloring material so as to evaporate this latter, wherein the improvement comprises: a photometric control device for directly sensing the transparency of the lens being colored to generate an electric current proportional to the luminous intensity of the lens being colored, means operable before heating said coloring material for causing said photometric control device to directly sense temporarily the transparency of a sample lens to generate a reference electric current, an element presettable according to said reference electric current and connected to an electric power source so as to generate an electric current proportional to the luminous intensity of the sample lens, a comparing circuit for comparing the current generated by said element so preset with the current generated by said photometric control device in sensing said lens being colored, and control means conditioned by said comparing circuit for controlling said heating means.
 2. An apparatus according to claim 1, wherein said element is formed of a potentiometer, and wherein said photometric control device comprises a photocell, said potentiometer being preset by hand upon sensing said sample lens throgh said photocell.
 3. An apparatus according to claim 2, wherein said operable means comprises a shifting member shiftable between two different positions, said member having a pair of seats, one of which is loaded with a transparent lens and the other with the sample lens said member when in one of said positions locating said transparent lens in correspondence with said photocell, when in the other of said positions locating said sample lens in correspondence with said photocell.
 4. An apparatus according to claim 1, wherein said comparing circuit Comprises a threshold circuit, which operates said control means to switch off said heating means.
 5. An apparatus according to claim 1, wherein said heating means comprises a resistor, and a variable transformer controlled by a wattmetric control circuit connected to said resistor for causing said transformer to supply a constant power to said resistor.
 6. An apparatus according to claim 5, comprising a reversible motor connected to said transformer and operable in a first direction to increase the power supplied by said transformer and in a second direction to decrease said supplied power, said wattmetric control circuit causing said motor to be rotated in said first direction, said wattmetric control circuit comprising a second comparing circuit for comparing a predetermined electric current with a current proportional to the electric power absorbed by said resistor to stop said motor when a predetermined power is absorbed by said resistor.
 7. An apparatus according to claim 6, wherein said second comparing circuit comprises a first threshold circuit for operting said motor in said first direction, said predetermined electric current being produced by a potentiometer preset so as to generate a corresponding threshold voltage when said predetermined power is absorbed.
 8. An apparatus according to claim 7, wherein said control means comprises a second threshold circuit to cause said motor to rotate in said second direction, said first and second threshold circuits operating said motor by means of two corresponding relays.
 9. An apparatus according to claim 1, comprising a pair of crucibles, one of which is loaded with said coloring material, the other of which is loaded with an antireflective material, a common support for carrying both said crucibles, and means for alternately shifting said crucibles in correspondence with said resistor.
 10. An apparatus according to claim 9, wherein said common support comprises a shaft rotatable stepwise 180* and a transverse bar carrying said crucibles at the two ends thereof and having the middle portion secured to said shaft.
 11. An apparatus according to claim 10, comprising a program control unit for sequentially controlling the operations of said motor and the stepwise rotation of said shaft, said control unit controlling also at least a vacuum pump and high voltage discharge means inside said chamber.
 12. Apparatus for coloring articles, for example lens for spectacles, by evaporating a coloring material under vacuum, comprisng a vacuum chamber where both an article to be colored and said coloring material are introduced, and heating means located inside said chamber for heating said coloring material so as to evaporate this latter, wherein the improvement comprises comparing means for comparing the intensity of coloration of the article being colored with a predetermined intensity of coloration, control means conditioned by said comparing means for controlling said heating means, said comparing means comprising a comparing circuit for comparing a current proportional to the luminous intensity of the article being colored and generated by a photometric control device, with a current generated by an element preset according to said predetermined intensity, said articles being made of glass, and a shifting member shiftable between two different positions, said member having a plurality of seats, one of which is loaded with a transparent article and another with the sample article, said member when in one of said positions locating said transparent article in correspondence with said photometric control device and when in the other of said positions locating said sample article in correspondence with said photometric control device, whereby said predetermined intensity may be determined by sensing the sample article through said photometric control device. 